Alleles

• Dominant allele


  An allele that will show in the phenotype even if there is only one copy present
• Recessive allele


  An allele that will only show in the phenotype if two copies are present (no dominant allele is present)
• Homozygous


  Having two copies of the same allele
• Heterozygous


  Having two different alleles
• Genotype


  The alleles present in an organism
• Phenotype


  The characteristics caused by an organism's alleles
• Normal human cells contain 23 pairs of chromosomes
• One chromosome in each pair comes from the father, the other from the mother
• Genes


  Sections of DNA on a chromosome
• Most characteristics are controlled by many genes acting together
• Alleles


  Different versions of a gene
• The earwax gene has two alleles
• The allele for wet earwax is dominant to the allele for dry earwax
• A person with genotype Ee has wet earwax phenotype because the dominant wet earwax allele is present
• A person with genotype ee has dry earwax phenotype because the recessive dry earwax allele is present
• Most characteristics are controlled by many genes acting together, not just single genes